1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery, a battery pack, a positive electrode, and a vehicle.
2. Description of the Related Art
A nonaqueous electrolyte battery in which the charging and the discharging operations are performed through migration of lithium ions between the negative electrode and the positive electrode, has been researched and developed as a high energy density battery. Lithium ions migrate from the positive electrode to the negative electrode in the charging operation, and from the negative electrode to the positive electrode in the discharging operation. A positive electrode active material mainly serves as a lithium source of lithium ions, which contribute to the charging/discharging operation.
A nonaqueous electrolyte battery has been commercialized in which lithium-transition metal composite oxide is used for the positive electrode active material, and carbonaceous material is used for the negative electrode active material. It is a common practice that Co, Mn, Ni or the like is used for a transitional metal component of the lithium-transition metal composite oxide.
In the case of the nonaqueous electrolyte battery, the initial charge/discharge efficiency of the negative electrode is lower than that of the positive electrode. Accordingly, in the initial charging/discharging cycle, the available electric capacity of the negative electrode is more greatly reduced than that of the positive electrode. In other words, the available electric capacity of the negative electrode is smaller than that of the positive electrode. As a result, as shown in FIG. 7, the negative electrode potential ascends to reach the discharge cutoff voltage of the battery. In other words, the discharge cutoff voltage of the battery depends on a variation of the negative electrode potential.
Recently, a nonaqueous electrolyte battery in which a titanium containing oxide is used for the negative electrode active material is attracting attention.
There is an approach that two or more kinds of active materials are used for the positive electrode. JP-A 2002-279989(KOKAI), for example, discloses a technique in which a mixture of phosphorus oxide, e.g., LiFePO4, and oxide, e.g., LiCoO2, is used for the positive electrode active material.